Although alumina-supported gold nanoparticles are poor catalysts for the oxidation of carbon monoxide, they have turned out to be promising candidates for the preferential oxidation of CO in hydrogen-rich streams (PrOx), as hydrogen apparently enhances the CO oxidation rate. The mechanism of this promotion effect is unclear. In this study, we carry out kinetic measurements on the PrOx reaction catalyzed by a 0.9% Au/Al2O3 catalyst, which is prepared by direct anionic exchange. We show that the apparent activation energy of the oxidation of CO is lower than that of the oxidation of H-2, whatever the hydrogen content in the feed. On the other hand, the hydrogen partial reaction order is higher in the oxidation of H-2 than in the oxidation of CO. Thus, the CO oxidation rate is significantly increased at low temperature by the introduction of only a small amount of hydrogen in the reactant mixture. At higher temperatures, the selectivity to CO2 decreases due to competition with the oxidation of H-2. Higher hydrogen concentrations cause the competition between CO and H-2 oxidations to start at lower temperatures. It is proposed that hydrogen reacts with oxygen to yield highly oxidizing intermediates that selectively react with CO as long as the energetic barrier to produce water from these intermediates is not crossed. (C) 2007 Elsevier B.V. All rights reserved.